CONFORMATIONS OF ALKANES AND CYCLOALKANES 55 3.26(a) Three isomers of Cs H contain two rings and have no alkyl substituents: (b) Five isomers of C6Hlo contain two rings and have no alkyl substituents Spirohexane Bicyclo[2.2. 0Jhexane Bicyclo[3. 1.0]hexane Bicyclo[2. 1. 1]hexane Cyclopropylcyclopropane 3.27 (a) The heat of combustion is highest for the hydrocarbon with the greatest number of carbons Thus, cyclopropane, even though it is more strained than cyclobutane or cyclopentane, has the lowest heat of combustion Cyclopentane Heat of combustion 3291 kJ/mol Cyclobutane Heat of combustion 2721 kJ/mol (650.3 kcal/mol) △ Cycloprop Heat of combustion 2091 kJ/mol (499.8 kcal/mol) A comparison of heats of combustion can only be used to assess relative stability when the ompounds are isomers. (b) All these compounds have the molecular formula CHi. They are isomers, and so the one with the most strain will have the highest heat of combustion 1, 1, 2, 2-Tetramethylcyclopropane Heat of combustion (high in angle strain; bonds are 4635 kJ/mol eclipsed; van der Waals strain (1107.9 kcal/mol) between cis methyl groups) cis-1, 2-Dimethylcyclopentane Cow angle strain; some torsional 4590 kJ/mol strain van der waals strain (1097. 1 kcal/mol) CH between cis methyl groups) Methylcyclohexane Heat of combustion CHs(minimal angle, torsional, and 4565 kJ/mol van der Waals strain) (1091. 1 kcal/mol) Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website3.26 (a) Three isomers of C5H8 contain two rings and have no alkyl substituents: (b) Five isomers of C6H10 contain two rings and have no alkyl substituents: 3.27 (a) The heat of combustion is highest for the hydrocarbon with the greatest number of carbons. Thus, cyclopropane, even though it is more strained than cyclobutane or cyclopentane, has the lowest heat of combustion. A comparison of heats of combustion can only be used to assess relative stability when the compounds are isomers. (b) All these compounds have the molecular formula C7H14. They are isomers, and so the one with the most strain will have the highest heat of combustion. H3C CH3 H3C CH3 1,1,2,2-Tetramethylcyclopropane (high in angle strain; bonds are eclipsed; van der Waals strain between cis methyl groups) Heat of combustion 4635 kJ/mol (1107.9 kcal/mol) H3C CH3 H H cis-1,2-Dimethylcyclopentane (low angle strain; some torsional strain; van der Waals strain between cis methyl groups) Heat of combustion 4590 kJ/mol (1097.1 kcal/mol) Methylcyclohexane (minimal angle, torsional, and van der Waals strain) Heat of combustion 4565 kJ/mol (1091.1 kcal/mol) CH3 Cyclopentane Heat of combustion 3291 kJ/mol (786.6 kcal/mol) Cyclobutane Heat of combustion 2721 kJ/mol (650.3 kcal/mol) Cyclopropane Heat of combustion 2091 kJ/mol (499.8 kcal/mol) Spirohexane Bicyclo[2.2.0]hexane Bicyclo[2.1.1]hexane Cyclopropylcyclopropane Bicyclo[3.1.0]hexane Spiropentane Bicyclo[2.1.0]pentane Bicyclo[1.1.1]pentane CONFORMATIONS OF ALKANES AND CYCLOALKANES 55 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website